The present invention relates to screws of novel construction, and more particularly to novel drilling and tapping screws.
Self-drilling and -tapping screws having drilling, tapping and fastening functions are widely used for assembling parts with ease and economically. In fact, various screws of this type have heretofore been proposed as disclosed in U.S. Pat. No. 3,463,045 (Japanese Patent Publication No. 2562/1972), U.S. Pat. No. 3,395,603 (Japanese Patent Publication No. 13139/1973), U.S. Pat. Nos. 3,738,043, 3,861,269, etc. Many of these proposals have been variously improved and found wide application in the art especially for specific uses.
However some of the screws heretofore proposed require a relatively uneconomical manufacturing method, while difficulties are encountered in the manufacture of screws of the type described since they are small-sized. Additionally there are limitations on the thickness of articles for which such screws are used. For example, these screws are usable for iron plates as thin as up to about 3 mm, because the cutting edge could break if used for larger thicknesses.
Self-drilling screws further involve some difficulty in the removal of the chips resulting from the cutting operation.